This invention pertains to apparatus and methods for providing an enhanced operation of a cracker and reformer using a naphtha feedstock using high-flux membranes to improve the composition of the feeds to each of the cracker and reformer. Full range naphtha feedstocks contain normal and non-normal paraffins. These feedstocks find numerous uses in petroleum refineries to produce useful petroleum and petrochemical products. One common use of naphtha feedstocks is as a feed to crackers, especially ethylene crackers, to produce olefins such as ethylene and propylene. Another utility is as a feed to a reformer to increase the octane rating of the naphtha such that it can be used in gasoline or to produce aromatics as chemical feedstocks.
Steam cracking, which is the thermal cracking of hydrocarbons in the presence of steam, is used commercially in large scale industrial units to produce ethylene and to a lesser extent propylene. These pyrolysis units are often charged a naphtha boiling range feed stream. The typical petroleum derived naphtha contains a wide variety of different hydrocarbon types including normal paraffins, branched paraffins, olefins, naphthenes, benzene, and alkyl aromatics. It is known in the art that paraffins are the most easily cracked and provide the highest yield of ethylene and that some compounds such as benzene are relatively refractory to the typical cracking conditions. It is also known that cracking normal paraffins results in a higher product yield than cracking isoparaffins. A paper entitled “Separation of Normal Paraffins from Isoparaffins” presented by I. A. Reddock, et al, at the Eleventh Australian Conference on Chemical Engineering, Brisbane, Sep. 4-7, 1983 discloses that the ethylene yield of a cracking unit can be increased if it is charged a C5-C9 stream of normal paraffins rather than a typical C5-C9 natural gasoline.
Adsorptive processes have been proposed to separate normal from non-normal paraffins in naphtha feedstocks. These processes usually use molecular sieves such as 5 A molecular sieve. See, for instance, U.S. Pat. No. 6,407,301. Not only do adsorptive separation processes require substantial capital but also many of the sorbents used have low tolerance for sulfur, nitrogen and oxygen-containing components. Thus, the entire naphtha stream must be subjected to pretreatment to remove deleterious components to the adsorptive separation. Alternatives to adsorptive separation are sought. However, due to the proximity of normal and non-normal boiling points of the components in a naphtha feedstock, distillation is not commercially practical.
U.S. Pat. No. 5,069,794 discloses microporous membranes containing crystalline molecular sieve material. At column 8, lines 11 et seq., potential applications of the membranes are disclosed including the separation of normal and branched paraffins. See also, U.S. Pat. No. 6,090,289, disclosing a layered composite containing molecular sieve that could be used as a membrane. Among the potential separations in which the membrane may be used that are disclosed commencing at column 13, line 6, include the separation of normal paraffins from branched paraffins. U.S. Pat. Nos. 6,156,950 and 6,338,791 discuss permeation separation techniques that may have application for the separation of normal paraffins from branched paraffins and describe certain separation schemes in connection with isomerization. US 2003/0196931 discloses a two-stage isomerization process for up-grading hydrocarbon feeds of 4 to 12 carbon atoms. The use of zeolite membranes is suggested as a suitable technique for separating normal molecules. See, for instance, paragraphs 0008 and 0032.
Although the use of membranes for the separation of normal from branched paraffins has been suggested, including in connection with certain isomerization processes, membranes have yet to find a practical, commercial application. Especially in a refinery involving large volumes of hydrocarbon feeds, extremely large membrane surface areas would have to be provided in order to achieve the sought separation of the normal paraffins. For instance, ZSM-5/Silicalite (MFI) membranes (a sieving membrane) available from NGK Insulators, Ltd., Japan, that have selectivity for the permeation of normal paraffins over branched paraffins, have a flux under operating conditions in the range of 0.1 to 1.0 milligram moles per second per square meter at a pressure differential of 15 to 500 kPa. U.S. Pat. No. 6,818,333 discloses thin zeolite membranes that are said to have a permeability of n-butane of at least 6·10−7 mol/m2 s·Pa and a selectivity of at least 250 of n-butane to isobutane. Bourney, et al., in WO 2005/049766 disclose a process for producing high octane gasoline using a membrane to remove, inter alia, n-pentane from an isomerized stream derived from the overhead of a deisohexanizer. In a computer simulation based upon the use of an MFI on alumina membrane, example 1 of the publication indicates that 5000 square meters of membrane surface area is required to remove 95 mass percent of n-pentane from the overhead from a deisohexanizer distillation column. At the flow rate of feed to the permeator (75000 kg/hr. having 20.6 mass percent n-pentane), the flux of n-pentane used in the simulation appears to be in the order of 0.01 gram moles/m2·s at 300° C.
Thus, the costs for commercially implementing such membrane separation systems render them not competitive with respect to an adsorption separation system even if the entire naphtha stream were treated to remove contaminants potentially deleterious to the sorbent.
For the purposes of the following discussion of the invention, the following membrane properties are defined.
Microporous
Microporous and microporosity refer to pores having effective diameters of between 0.3 to 2 nanometers.
Mesoporous
Mesoporous and mesoporosity refer to pores having effective diameters of between 2 and 50 nanometers.
Macroporous
Macroporous and macroporosity refer to pores having effective diameters of greater than 50 nanometers.
Nanoparticle
Nanoparticles are particles having a major dimension up to 100 nanometers.
Molecular Sieves
Molecular sieves are materials having microporosity and may be amorphous, partially amorphous or crystalline and may be zeolitic, polymeric, metal, ceramic or carbon.
Sieving Membrane
Sieving membrane is a composite membrane containing a continuous or discontinuous selective separation medium containing molecular sieve barrier. A barrier is the structure that exists to selectively block fluid flow in the membrane. In a continuous sieving membrane, the molecular sieve itself forms a continuous layer that is sought to be defect-free. The continuous barrier may contain other materials such as would be the case with mixed matrix membranes. A discontinuous sieving membrane is a discontinuous assembly of molecular sieve barrier in which spaces, or voids, exist between particles or regions of molecular sieve. These spaces or voids may contain or be filled with other solid material. The particles or regions of molecular sieve are the barrier. The separation effected by sieving membranes may be on steric properties of the components to be separated. Other factors may also affect permeation. One is the sorptivity or lack thereof by a component and the material of the molecular sieve. Another is the interaction of components to be separated in the microporous structure of the molecular sieve. For instance, for some zeolitic molecular sieves, the presence of a molecule, say, n-hexane, in a pore, may hinder 2-methylpentane from entering that pore more than another n-hexane molecule. Hence, zeolites that would not appear to offer much selectivity for the separation of normal and branched paraffins solely from the standpoint of molecular size, may in practice provide greater selectivities of separation.
C6 Permeate Flow Index
The permeability of a sieve membrane, i.e., the rate that a given component passes through a given thickness of the membrane, often varies with changes in conditions such as temperature and pressure, absolute and differential. Thus, for instance, a different permeation rate may be determined where the absolute pressure on the permeate side is 1000 kPa rather than that where that pressure is 5000 kPa, all other parameters, including pressure differential, being constant. Accordingly, a C6 Permeate Flow Index is used herein for describing sieving membranes. The C6 Permeate Flow Index for a given membrane is determined by measuring the rate (gram moles per second) at which a substantially pure normal hexane (preferably at least 95 wt-% normal hexane) permeates the membrane at approximately 150° C. at a retentate side pressure of 1000 kPa absolute and a permeate-side pressure of 100 kPa absolute. The C6 Permeate Flow Index reflects the permeation rate per square meter of retentate-side surface area but is not normalized to membrane thickness. Hence, the C6 Permeate Flow Index for a given membrane will be in the units of gram moles of normal hexane permeating per second per square meter of retentate-side membrane surface area.
C6 Permeate Flow Ratio
The C6 Permeate Flow Ratio for a given sieve membrane is the ratio of the C6 Permeate Flow Index to an i-C6 Permeate Flow Index wherein the i-C6 Permeate Flow Index is determined in the same manner as the C6 Permeate Flow Index but using substantially pure dimethylbutanes (regardless of distribution between 2,2-di-ethylbutane and 2,3-dimethylbutane) (preferably at least 95 mass-% dimethylbutanes).